Generally, a ring rolling process is a process which machines a seamless ring in a continuous manner into a predetermined size, thus producing a product, that is, a rolled ring product. Such ring rolling processes are used to manufacture ring parts used in a variety of fields, for example, power generation equipment, chemical plants, gas turbines, jet engines, etc.
Compared to a ring forging process which is different from a rolling process, advantages of the ring rolling process include that the working speed is rapid, the temperature can be maintained, the production yield can be enhanced, and so on. Particularly, in the case of a rolled ring product that is manufactured by a ring rolling process, the grain flow line is continuously formed in the circumferential direction of the product, thus providing superior characteristics.
FIG. 1 is of views showing an entire ring rolling process. A method of manufacturing a rolled ring product with the ring rolling process will be explained with reference to FIG. 1. At step S1, an initial billet 1 with, for example, a cylindrical structure, is prepared by cutting off a raw billet to an appropriate size using gas cutting or a machine saw.
Subsequently, at step S2, a heating furnace 2 heats the initial billet 1 to the desired temperature. At step S3, the heated initial billet 1 is transferred to a forging press 3.
A mold of the forging press 3 that has been preheated upset-forges the heated initial billet 1, thus pressing the initial billet 1 in the axial direction, at step S4.
Thereafter, at step S5, a punch 4 pierces an intermediate product 1a that has been compressed by upset-forging the initial billet 1, thus forming a hollow blank 9.
At step S6, a ring rolling machine subsequently ring-rolls the blank 9. The ring rolling machine includes a main roll 5 which presses a circumferential outer surface of the blank 9, a pressure roll 6 which presses a circumferential inner surface of the blank 9, an upper axial roll 7 which presses an upper surface of the blank 9, a lower axial roll 8 which presses a lower surface of the blank 9, and a plurality of guide rolls 10 which rotatably support the circumferential outer surface of the blank 9. This ring rolling process produces a rolled ring product 11 into a predetermined shape, at step S7.
Particularly, a flange, which is used to connect tubes that form the framework of a wind tower, is typically manufactured by such a ring rolling process. FIG. 2 is a sectional view of a typical flange for wind towers.
As shown in FIG. 2, the flange 10 for wind towers includes a connection part 10a that protrudes from the body of the flange 10 and is used when welding a corresponding tube to the flange 10. Producing the flange 10 includes the ring rolling process manufacturing a ring having a rectangular cross-section as illustrated in FIG. 1, and post-processing the ring, thus producing a final product.
FIG. 3 is a sectional view of a ring-rolled product having a depression. Recently, as shown in FIG. 3, a method is used in which an intermediate product having a depression 12 in a circumferential inner surface thereof is formed, the intermediate product is cut into two parts at a medial portion thereof corresponding to the depression 12, and then each of the two parts is post-processed, thus forming a final product 10.
Representative examples of the above conventional technique were proposed in Korean Patent Application No. 10-2009-0131482 (filed on Dec. 28, 2009: Semi-finished ring rolling machine and method of manufacturing semi-finished ring using the same), Patent Application No. 10-2010-0007954 (filed on Jan. 28, 2010: Apparatus and method for manufacturing asymmetric large ring), etc.
However, to form such a product having a depression, a protrusion must be provided on the pressure roll.
FIGS. 4A and 4B are sectional views comparing the shapes of blanks depending on the presence of the protrusion.
FIG. 4A illustrates the case of a typical pressure roll having no protrusion. FIG. 4B illustrates the case of a pressure roll provided with a protrusion. As shown in FIGS. 4A and 4B, compared to the case (FIG. 4A) of the typical pressure roll, a disadvantage of the case (FIG. 4B) of the pressure roll provided with the protrusion is that the inner diameter of the blank is increased (d1<d2), so that the diameter of a punch that is used to pierce the blank must also be increased, and the material utilization ratio is reduced.